METAPELITES AND GRANITIC INTRUSIONS OF THE ANGEL LAKE CIRQUE, RUBY MOUNTAINS-EAST HUMBOLDT RANGE METAMORPHIC CORE COMPLEX: A GEOCHRONOLOGICAL AND TEXTURAL STUDY OF ZIRCON AND MONAZITE

The Ruby Mountains-East Humboldt Range metamorphic core complex shows a complex history of magmatism and metamorphism. The Angel Lake area in the northern part of the core complex exposes the oldest rocks in the complex. Textural analyses of minerals and rocks integrated with in situ geochronology aids understanding relationships between deposition, metamorphism, deformation, partial melting and magmatism.

Monazite from gneissic samples are dominated by patchy zoning with sharp boundaries between domains, or cores and patchy rim textures. Geochronology shows four populations: 1) an inherited ~2440 Ma population; 2) a population between 75-90 Ma with a peak at 79 Ma; 3) a population between 30-40 Ma with peaks at 38 Ma and 32 Ma; and 4) a population between 25-30 Ma. Monazites from leucogranites show oscillatory and patchy zoning and U-Pb data shows two populations: one at 30-33 Ma, and a second at 24-26 Ma. Zircons from biotite monzogranites have complex internal zoning patterns that include oscillatory and mottled domains with core ± mantle + rim relationships. Monazites from monzogranites have igneous growth textures, and complex curvilinear domains with sharp boundaries. Calculated age populations are 31-34 Ma, 74-88 Ma (with a peak at 78-81 Ma), 152-172 Ma, and a series of >1.0 Ga analyses (n=8). Based on textural constraints, the >1.0 Ga and 152-172 Ma populations are inherited, and the attributed age of primary crystallization is ~79 Ma. The younger populations of 30-40 Ma and 24-30 Ma are interpreted as metamorphism of the monzogranite.

Monzogranite emplacement was coeval with crystallization of pegmatitic leucogranites in Lamoille Canyon and may have inherited zircon and monazite from units such as the two-mica granite of Daley Canyon and biotite granite of Seitz Creek. The Angel Lake cirque records the same period of magmatic quiescence recorded in Lamoille Canyon (40-70 Ma). Thermodynamic modeling of metapelite compositions agree with the production of small volumes of felsic melt by fluid-fluxed isothermal decompression, which is compatible with the tectonic framework of core complex exhumation at that time. Partial melts in deeper structural levels may have contributed to larger volumes of leucogranites observed in structurally higher levels of the core complex.